Infrared (IR) detectors are often utilized to detect fires, overheating machinery, planes, vehicles, people, and any other objects that emit thermal radiation. Infrared detectors are unaffected by ambient light conditions or particulate matter in the air such as smoke or fog. Thus, infrared detectors have potential use in night vision and when poor vision conditions exist, such as when normal vision is obscured by smoke or fog. IR detectors are also used in non-imaging applications such as radiometers, gas detectors, and other IR sensors.
A variety of infrared detector types have been developed in the past. Many include a focal plane array (FPA) that includes a substrate with a plurality of detector elements that each correspond to a respective pixel. The substrate includes integrated circuitry which is electrically coupled to other components of the detector elements, and which is commonly known as a read out integrated circuit (ROIC).
Infrared detectors generally operate by detecting the differences in thermal radiance of various objects in a scene. That difference is converted into an electrical signal which is then processed. Microbolometers are infrared radiation detector elements that are fabricated on a substrate material using traditional integrated circuit fabrication techniques. Microbolometer detector arrays consist of thin, low thermal mass, thermally isolated, temperature-dependent resistive membrane pixel structures. The microbolometer pixel structures are suspended over silicon ROIC wafers by long thermal isolation legs in a resonant absorbing quarter-wave cavity design.
FIG. 1A illustrates a conventional prior art infrared detector element 100 with a microbolometer pixel membrane structure 112 that includes thin (1000 A to 4000 A) thermally absorbing membrane material of a resistive material like amorphous silicon (a-Si); amorphous silicon geranium (a-SiGe); or Vanadium Oxide together with an infrared absorbing thin metal like Ti, TiAl; TiN; or Ni and supporting silicon nitride, silicon oxide; or silicon oxynitride. The microbolometer pixel membrane structure 112 is suspended approximately 2 microns above silicon semiconductor substrate 110 by long thermal isolation legs 116 that are electrically connected to the ROIC within semiconductor substrate 110 by metal interconnects (typically TiW or Aluminum) via aluminum input pads 114. Disposed on the surface of the supporting substrate 110 and ROIC is a metal reflector 118 (typically Aluminum) to form a resonant cavity structure to enhance infrared absorptance in the membrane of the suspended microbolometer pixel structure 112. For infrared applications, pixel size typically ranges from 12 um×12 um up to 100 um×100 um.
The primary factors affecting response time and sensitivity of microbolometers are thermal mass and thermal isolation. Microbolometer response time is the time necessary for a detector element to absorb sufficient infrared radiation to alter an electrical property, such as resistance, of the detector element and to dissipate the heat resulting from the absorption of the infrared radiation. Microbolometer sensitivity is determined by the amount of infrared radiation required to cause a sufficient change in an electrical property of the microbolometer detector element. Microbolometer response time is inversely proportional to both thermal mass and thermal isolation. Thus, as thermal mass increases, response time becomes slower since more infrared energy is needed to sufficiently heat the additional thermal mass in order to obtain a measurable change in an electrical property of the microbolometer detector element. Therefore, it is generally desirable to make microbolometer pixels that are low in mass in order to achieve a short thermal time constant, while at the same time maximizing absorption of radiation.
In the past, material has been removed from a microbolometer pixel membrane structure in a square or rectangular grid pattern to reduce microbolometer pixel mass and to shorten thermal time constant while maintaining good radiation absorption characteristics. FIG. 1B illustrates a simplified perspective view of the prior art suspended microbolometer detector pixel structure 112 of FIG. 1 that includes conductive element portions 106 that are oriented both parallel and orthogonal to the path of current flow between electrical contacts 102 and 104. As illustrated, openings in the form of square holes 111 are defined by material removed between the electrical contacts 106 to reduce the microbolometer pixel mass. The holes are dimensioned to be small compared to the radiation wavelength of interest. Microbolometer pixels fabricated with this structure are sometimes called diffractive resonant cavities (DRCs).